Verifiability ("VER") means that all content on Wikipedia can be verified in a reliable publication. This can be the source from which the information is derived, but also other literature. Something should not be included just because it is 'true'. Own opinions and/or speculations are not allowed. If various views on a topic can be found in reliable publications, the article should reflect these diverse views, with each view given space according to the relative importance attributed to this view in the literature, something that should also be verifiable .
The 'verifiability' ('VER'), 'neutral point of view' ('NPOV') and 'no original research' ('GOO') pages describe the core principles that Wikipedia content must adhere to. They complement each other and cannot be interpreted separately. Users should familiarize themselves with these pages. The principles discussed therein can never be set aside, not even by consensus.
Everything in Wikipedia's main namespace should be found in publications that are not only reliable and authoritative, but also relevant to the subject. Wikipedia itself never counts as a reliable publication (in some cases material in Wikisource or an image in Commons can be used as a reliable source; for example the demolition of a building).
Burden of proof
Throughout Wikipedia, the burden of proof for the correctness of material lies with the contributor (each user is responsible for his own contributions) and with the person who restores the material after a deletion. If something cannot be substantiated, it should be removed.
To make it easy to verify the information in an article, reference can be made to the source. Citations or references are not required, except for content that is controversial. After all, much is generally known ("The capital of France is Paris.") or can be looked up in many publications ("the acceleration of gravity on the moon is 1.6 m/s²"). And a bibliography or musical notes can also be distracting.
NB. When quoting from copyrighted work, it is legally required to report the source, and moreover also the creator of the work, and there are further obligations (the so-called right to quote). These obligations apply everywhere, including Wikipedia.
In itself it does not matter whether a publication is easily available; it is primarily about the reliability and appropriateness of a publication. A given website may be easily accessible, but it is therefore no better than, or even comparable to, a reference work mainly found in university libraries. "I can't find this on the internet" is therefore not a valid argument for removing information from Wikipedia. For very old sources — which are in the public domain — the publication can be made available to everyone by uploading a scan to Wikimedia Commons.
Historical sources must be placed in the context and perception of the time. An interpretation of historical events must be supported by modern sources, otherwise it becomes original research.
Wikipedia founder Jimmy Wales writes May 16, 2006 on the English-language Wikipedia mailing list about the importance of verifiability on Wikipedia. He emphasizes that the lack of information is preferable to misleading or incorrect information.
What to do when in doubt?
There can be many reasons for doubt. The purpose of an encyclopedia is to bring together and present information, including information for which